Advancements in treatment for childhood Acute Lymphoblastic Leukemia (ALL) have resulted in a growing cohort of survivors. Many survivors of childhood ALL do not meet recommendations for physical activity or healthy diet, and are at risk for obesity and overweight as they move into adulthood. These lifestyle behaviors have the potential to complicate health problems for which these survivors are already at risk. To date, there are few efficacious interventions available to assist this group of cancer survivors wishing to adopt healthier lifestyles. Because survivors of ALL are geographically dispersed and relatively small in numbers, interventions will need to be developed to be delivered at a distance. The primary aim of this proposal is the development and feasibility testing of an innovative intervention designed to promote healthy dietary intake, increase physical activity, and prevent weight gain that can be delivered using smarthphone technology. The intervention will be developed for adolescent survivors of childhood ALL ages 13-19 years who are at least 2 years off treatment. The intervention will include the use of a specialized, study-designed smartphone application (app), weekly supportive counseling, and materials for parents to help them support their child as they take part in the intervention. The app will include tools for near effortless self-monitoring of diet, physical activity, and weight; motivational messages and the use of a virtual token-economy to encourage participation; and a social-networking component that will allow participants to support one another. Elements of social- cognitive theory will be used to develop the app and intervention. A formative phase focused on intervention and software development will be followed by a pilot test of the intervention among 40 adolescents randomized to either a wait-list control (WLC) arm or the intervention arm. Outcomes will include quality of dietary intake, physical activity, body-mass index and social cognitive mediators of health behavior change. The investigative team has expertise in areas of pediatric psychology, pediatric hematology-oncology, obesity epidemiology/prevention, and computer science and thus, is well poised to undertake the proposed research. The successful completion of this proposal will set the stage for a larger multi-center randomized controlled trial in which we will evaluate the efficacy and cost-effectiveness of our intervention.